Medical devices, designed to be implanted in a patient's body, are typically operated by means of electrical power. Such implantable medical devices include electrical and mechanical stimulators, motors, pumps, etc, which are designed to support or stimulate various body functions. Electrical power can be supplied to such an implanted medical device from a likewise implanted battery or from an external energy source that can supply any needed amount of electrical power intermittently or continuously without requiring repeated surgical operations.
An external energy source can transfer wireless energy to an implanted internal energy receiver located inside the patient and connected to the medical device for supplying received energy thereto. So-called TET (Transcutaneous Energy Transfer) devices are known that can transfer wireless energy in this manner. Thereby, no leads or the like penetrating the skin need to be used for connecting the medical device to an external energy source, such as a battery.
A TET device typically comprises an external energy source, i.e. external to the patient in whose body the implanted device is located, including a primary coil adapted to inductively transfer any amount of wireless energy by inducing a voltage in a secondary coil of an internal energy receiver which is implanted preferably just beneath the skin of a patient. The highest transfer efficiency is obtained when the primary coil is positioned close to the skin adjacent to and in alignment with the secondary coil, i.e. when a symmetry axis of the primary coil is parallel to that of the secondary coil.
Typically, the amount of energy required to operate an implanted medical device may vary over time depending on the operational characteristics of the device. For example, the device may be designed to switch on and off at certain intervals, or otherwise change its behavior, in order to provide a suitable electrical or mechanical stimulation, or the like. Such operational variations will naturally result in corresponding variations with respect to the amount of required energy.
Furthermore, the position of the external energy source relative to the implanted internal energy receiver is a factor that affects the efficiency of the energy transfer, which highly depends on the distance and relative angle between the source and the receiver. For example, when primary and secondary coils are used, changes in coil spacing result in a corresponding variation of the induced voltage. During operation of the medical device, the patient's movements will typically change the relative spacing of the external source and the internal receiver arbitrarily such that the transfer efficiency greatly varies.
If the transfer efficiency becomes low, the amount of energy supplied to the medical device may be insufficient for operating the device properly, so that its action must be momentarily stopped, naturally disturbing the intended medical effect of the device.
On the other hand, the energy supplied to the medical device may also increase drastically, if the relative positions of the external source and the internal receiver change in a way that unintentionally increases the transfer efficiency. This situation can cause severe problems since the implant cannot “consume” the suddenly very high amount of supplied energy. Unused excessive energy must be absorbed in some way, resulting in the generation of heat, which is highly undesirable. Hence, if excessive energy is transferred from the external energy source to the internal energy receiver, the temperature of the implant will increase, which may damage the surrounding tissue or otherwise have a negative effect on the body functions. It is generally considered that the temperature in the body should not increase more than three degrees to avoid such problems.
For example, U.S. Pat. No. 5,995,874 discloses a TET system in which the amount of transmitted energy from a primary coil is controlled in response to an indication of measured characteristics of a secondary coil, such as load current and voltage. The transmitted energy can be controlled by varying the current and voltage in the primary coil, transmission frequency or coil dimensions. In particular, a change is effected in the saturation point of the magnetic field between the coils, in order to adjust the power transfer efficiency. However, it is not likely that this solution will work well in practice, since a saturation point in the human tissue would not occur, given the magnetic field levels that are possible to use. Moreover, if the energy transmission must be increased considerably, e.g. to compensate for losses due to variations in alignment and/or spacing between the coils, the relatively high radiation generated may be damaging or unhealthy or unpleasant to the patient, as is well known.
An effective solution is thus needed for more accurately transferring energy to an implanted medical device to ensure proper operation thereof, as well as optimizing the cooperation between the external energy source and the internal energy receiver in general.